EXAMPLE 1

84.9 ml of sponge zirconium that was procured from the market (prepared by screening crushed product to 6 to 32 mesh, with 99.5% purity) was packed in a quartz-made cleaning column having 19 mm inside diameter and 400 mm length. Then, helium (He) gas containing 1% nitrogen trifluoride (NF.sub.3) was passed through the column at a total flow rate of 170 ml/min, that is, a space linear velocity (LV) of 1 cm/sec at 250 C. under atmospheric pressure, and the column outlet gas was analyzed for NF.sub.3 by gas chromatography (lower limit of detectable range being 10 ppm). As a result, NF.sub.3 was not detected. Subsequently, the gas was passed through the column for further 3 hours'. As a result, there was not observed break through nor the formation of harmful byproduct such as NO.sub.x.

EXAMPLE 2

28.3 ml of the sponge zirconium same as that used in Example 1 was packed in a quartz-made cleaning column having 19 mm inside diameter and 400 mm length. Then, He gas containing 1% NF.sub.3 was passed through the column at a total flow rate of 85 ml/min, that is, a space linear velocity (LV) of 0.5 cm/sec at room temperature under atmospheric pressure, and after 20 minutes the column outlet gas was analyzed to determine NF.sub.3 concentration by gas chromatography (lower limit of detectable range being 10 ppm). Subsequently, the gas temperature was raised by increments of 50 every increment to determine NF.sub.3 concentration in the column outlet gas by gas chromatography and thereby obtain NF.sub.3 decomposition efficiency at each incremental temperature. By plotting the decomposition efficiency thus obtained against the temperature, the lower limit of the temperature at which the NF.sub.3 decomposition efficiency exceeded 90% was obtained by means of interpolation. The result was 280

Thereafter, 8.5 ml of the same sponge zirconium as the cleaning agent was further packed in the column. Subsequently He gas was passed through the column at a total flow rate of 500 ml/min, while the gas temperature was raised to 300 passed through the column at a total flow rate of 509 ml/min, that is, a LV of 3 cm/sec at 300 outlet gas was monitored by means of an NF.sub.3 detector available on the market (TG-4100 TA, produced by Bionics Instruments Co., Ltd.) to determine the break through time by regarding the point at which NF.sub.3 concentration in the outlet gas reached 10 ppm as the break through point.

The cleaning capability (throughput of NF.sub.3 (L) per 1L of Zr) was obtained by calculation from the result thus obtained. The result was 711L/L. In addition in order to check byproduct formation, nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2) in the exhaust gas were measured prior to the break through by means of the detecting tubes (for separating and analyzing nitrogen oxides, lower limit of detectable range being 1 ppm for NO and 0.5 ppm for NO.sub.2, produced by Gastech Corp.). As a result, nitrogen oxide was not detected.

EXAMPLE 3

28.3 ml of cleaning agent comprising Zr-Fe alloy procured from the market (80% by weight of Zr with the balance Fe) which had been crushed and screened to 10 to 32 mesh was packed in a quartz-made cleaning column having 19 mm inside diameter and 400 mm length. Then He gas containing 1% NF.sub.3 was passed through the column at a total flow rate of 85 ml/min, that is, a space linear velocity (LV) of 0.5 cm/sec at room temperature under atmospheric pressure, and after 20 minutes the column outlet gas was analyzed to determine NF.sub.3 concentration by gas chromatography (lower limit of detectable range being 10 ppm). Subsequently, the gas temperature was raised by increments of 50 maintained for 10 minutes at every increment to determine NF.sub.3 concentration in the column outlet gas by gas chromatography and thereby obtain NF.sub.3 decomposition efficiency at each incremental temperature. By plotting the decomposition efficiency thus obtained against the temperature, the lower limit of the temperature at which the NF.sub.3 decomposition efficiency exceeded 90% was obtained by means of interpolation. The result is given in Table 1.

Thereafter, 8.5 mol of the same alloy as the cleaning agent was further packed in the column. Subsequently He gas was passed through the column at a total flow rate of 500 ml/min, while the gas temperature was raised to 200 through the column at a total flow rate of 509 ml/min, that is, a LV of 3 cm/sec under atmospheric pressure, while the outlet gas was monitored by means of an NF.sub.3 detector available on the market (TG-4100 TA, produced by Bionics Instruments Co., Ltd.) to determine the break through time by regarding the point at which NF.sub.3 concentration in the outlet gas reached 10 ppm as the break through point.

The cleaning capability (throughput of NF.sub.3 (L) per 1L of Zr-alloy) was obtained by calculation from the result thus obtained. In addition in order to check byproduct formation, nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2) in the exhaust gas were measured prior to the break through by means of the detecting tubes (for separating and analyzing nitrogen oxides, lower limit of detectable range being 1 ppm for NO and 0.5 ppm for NO.sub.2, produced by Gastech Corp.). The results are given in Table 2.

EXAMPLE 4 TO 6

Sponge zirconium and reduced iron each procured from the market in a total amount of about 500 g were blended in different compositions (90%, 50% or 40% by weight of Zr and the balance iron) and then melted by electron beam twice repeatedly to afford Zr-Fe alloys, which were crushed in a ball mill and screened to 14 to 20 mesh to prepare various alloys each having a different composition from one another.

Subsequently, the procedure in Example 3 was repeated to carry out cleaning experiment except that each of the above-prepared alloys was used as the cleaning agent and the cleaning temperature was altered, respectively. The results are given in Table 1 and Table 2.

COMPARATIVE EXAMPLE 1

The procedure in Example 3 was repeated to carry out cleaning experiment except that there was used as the cleaning agent iron wires procured from the market and cut into a length of 5 to 10 mm. The results are given in table 1 and Table 2.

              TABLE 1______________________________________  Alloy compo-           Lower limit of temperature (  sition (wt %)           attaining ≧90% decomposition  Fe    Zr     efficiency______________________________________Example 3    20      80     195Example 4    10      90     195Example 5    50      50     245Example 6    60      40     250Comparative    100      0     325Example 1______________________________________

              TABLE 2______________________________________  Cleaning   Cleaning capability                           Nitrogen  temperature (             (L/L)         oxide______________________________________Example 3    200           594          NDExample 4    200          >100          NDExample 5    250          >100          NDExample 6    280          >100          NDComparative    300            3           NDExample 1    400            7           ND______________________________________

EXAMPLES 7 TO 15

Sponge zirconium and copper or silver each procured from the market in a total amount of about 500 g were blended in different compositions (40%, 50%, 75% or 90% by weight of Zr and the balance copper or silver) and then melted by an electron beam twice repeatedly to afford Zr-Cu or Zr-Ag alloys, which were crushed in a ball mill and screened to 14 to 20 mesh to prepare various alloys each having a different composition. Then, 28.3 ml of each of the alloys was packed in a quartz-made cleaning column having 19 mm inside diameter and 400 mm length. Then, helium (He) gas containing 1% NF.sub.3 was passed through the column at a total flow rate of 85 ml/min, that is, a space linear velocity (LV) of 0.5 cm/sec at room temperature under atmospheric pressure, and after 20 minutes the column outlet gas was analyzed to determine NF.sub.3 concentration by gas chromatography (lower limit of detectable range being 10 ppm). Subsequently, the gas temperature was raised by increments of 100 C., while the temperature was maintained for 10 minutes at every increment to determine NF.sub.3 concentration in the column outlet gas by gas chromatography and thereby obtain NF.sub.3 decomposition efficiency at each incremental temperature. By plotting the decomposition efficiency thus obtained against the temperature, the lower limit of the temperature at which the NF.sub.3 decomposition efficiency exceeded 90% was obtained by means of interpolation. The result is given in Table 3.

Thereafter, 8.5 ml of each of the alloys as the cleaning agent was further packed in a column. Subsequently He gas was passed through the column at a total flow rate of 500 ml/min, while the gas temperature was raised to the temperature as shown in Table 2. Thereafter, He gas containing 2% NF.sub.3 was passed through the column at a total flow rate of 509 ml/min, that is, a LV of 3 cm/sec under atmospheric pressure, while the outlet gas was monitored by means of an NF.sub.3 detector available on the market (TG-4100 TA, produced by Bionics Instruments Co., Ltd.) to determine the break through time by regarding the point at which NF.sub.3 concentration in the outlet gas reached 10 ppm as the break through point.

The cleaning capability (throughput of NF.sub.3 (L) per 1L of Zr-alloy) was obtained by calculation from the result thus obtained. In addition in order to check byproduct formation, nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2) in the exhaust gas were measured prior to the break through by means of the detecting tubes (for separating and analyzing nitrogen oxides, lower limit of detectable range being 1 ppm for NO and 0.5 ppm for NO.sub.2, produced by Gastech Corp.). The result are given in Table 4.

COMPARATIVE EXAMPLES 2 TO 3

The procedure in Example 3 was repeated to carry out cleaning experiment except that there were used as the copper source, copper wires procured from the market and cut into a length of 5 to 10 mm. The results are given in Table 3 and Table 4.

              TABLE 3______________________________________  Alloy compo-            Lower limit of temperature (  sition (wt %)            attaining ≧90% decomposition  Zr  Cu     Ag     efficiency______________________________________Example 7    40    60      0   235Example 8    50    50      0   235Example 9    75    25      0   220Example 10    90    10      0   225Example 11    40     0     60   260Example 12    50     0     50   255Example 13    75     0     25   235Example 14    90     0     10   250Example 15    75    15     10   225Comparative     0    100     0   335Example 2Comparative     0     0     100  430Example 3______________________________________

              TABLE 4______________________________________  Cleaning   Cleaning capability                           Nitrogen  temperature (             (L/L)         oxide______________________________________Example 7    280          >100          NDExample 8    250          >100          NDExample 9    230           568          NDExample 10    230          >100          NDExample 11    280          >100          NDExample 12    280          >100          NDExample 13    250          >100          NDExample 14    260          >100          NDExample 15    250          >100          NDComparative    350            19          NDExample 2Comparative    500            8           NDExample 3______________________________________

EXAMPLES 16 TO 29

Sponge zirconium and nickel, cobalt or manganese each procured from the market in a total amount of about 500 g were blended in different compositions (40%, 50%, 75% or 90% by weight of Zr and the balance Ni, Co or Mn) and then melted by an electron beam twice repeatedly to afford Zr-Ni, Zr-Co or Zr-Mn alloys, which were crushed in a ball mill and screened to 14 to 20 mesh to prepare various alloys as cleaning agent each having a different composition from one another. Then 28.3 ml of each of the alloys was packed in a quartz-made cleaning column having 19 mm inside diameter and 400 mm length. Each of the Zr-alloys as the cleaning agent was tested in the same manner as in Example 7 to determine the temperature attaining 90% decomposition efficiency, cleaning capability and nitrogen oxide concentration. The results are given in Table 5.

COMPARATIVE EXAMPLES 4 TO 6

The procedure in Example 16 was repeated to carry out cleaning experiment except that there was used as the cleaning agent, Ni, Co or Mn procured from the market and screened to 6 to 32 mesh. The results are given in Table 5 and Table 6.

              TABLE 5______________________________________                  Lower limit of tem-                  perature (                  taining ≧90%  Alloy composition (wt %)                  decomposition  Zr  Ni      Co      Mn    efficiency______________________________________Example 16    40    60      0     0     240Example 17    50    50      0     0     220Example 18    75    25      0     0     205Example 19    90    10      0     0     205Example 20    40    0       60    0     260Example 21    50    0       50    0     240Example 22    75    0       25    0     225Example 23    90    0       10    0     225Example 24    40    0       0     60    265Example 25    50    0       0     50    245Example 26    75    0       0     25    245Example 27    90    0       0     10    240Example 28    75    15      10    0     210Example 29    90    5       0     5     215Comparative     0    100     0     0     290Example 4Comparative     0    0       100   0     315Example 5Comparative     0    0       0     100   340Example 6______________________________________

              TABLE 6______________________________________  Cleaning   Cleaning capability                           Nitrogen  temperature (             (L/L)         oxide______________________________________Example 16    280          >100          NDExample 17    250          >100          NDExample 18    210           504          NDExample 19    210          >100          NDExample 20    280          >100          NDExample 21    250          >100          NDExample 22    240          >100          NDExample 23    240          >100          NDExample 24    280          >100          NDExample 25    280          >100          NDExample 26    250          >100          NDExample 27    250          >100          NDExample 28    210          >100          NDExample 29    215          >100          NDComparative    300            17          NDExample 4Comparative    330            10          NDExample 5Comparative    350            11          NDExample 6______________________________________

EXAMPLES 30 TO 51

Sponge zirconium and magnesium, calcium, zinc, aluminum, lanthanum or cerium each procured from the market in a total amount of about 500g were blended in different compositions (40%, 50%, 75% or 90% by weight of Zr and the balance Mg, Ca, Zn, Al, La or Ce) and then melted by an electron beam twice repeatedly to afford Zr-Mg, Zr-Ca, Zr-Zn, Zr-Al or Zr-Ce alloys, which were crushed in a ball mill and screened to 14 to 20 mesh to prepare various alloys as cleaning agent each having a different composition from one another. Each of the cleaning agents was tested in the same manner as in Example 7 to determine the temperature attaining 90% decomposition efficiency, cleaning capability and nitrogen oxide concentration. The results are given in Table 7 and Table 8.

COMPARATIVE EXAMPLES 7 TO 11

The procedure in Example 30 was repeated to carry out cleaning experiment except that there was used as the cleaning agent, sand magnesium, granular calcium, granular zinc (1 to 2 mm size), granular aluminum (2 to 3 mm size) or granular cerium (1 to 2 mm size). The results are given in Table 7 and Table 8.

              TABLE 7______________________________________  Alloy compo-              Lower limit of temperature  sition (wt %)              (  Zr  other metal composition efficiency______________________________________Example 30    40    Mg      60    225Example 31    50    Mg      50    225Example 32    75    Mg      25    190Example 33    90    Mg      10    195Example 34    40    Ca      60    230Example 35    50    Ca      50    230Example 36    75    Ca      25    200Example 37    90    Ca      10    210Example 38    40    Zn      60    245Example 39    50    Zn      50    215Example 40    75    Zn      25    205Example 41    90    Zn      10    200Example 42    40    Al      60    240Example 43    50    Al      50    240Example 44    75    Al      25    215Example 45    90    Al      10    210Example 46    40    La      60    245Example 47    50    La      50    240Example 48    75    La      25    230Example 49    90    La      10    240Example 50    40    Ce      60    235Example 51    50    Ce      50    245Example 52    75    Ce      25    215Example 53    90    Ce      10    225Example 54    75    Mg 15   Al 10 195Example 55    90    Mg 5    Zn 5  195Comparative      0   Mg      100   270Example 7Comparative     0    Ca      100   280Example 8Comparative     0    Zn      100   340Example 9Comparative     0    Al      100   465Example 10Comparative     0    La      100   320Example 11Comparative     0    Ce      100   310Example 12______________________________________

              TABLE 8______________________________________  Cleaning   Cleaning capability                           Nitrogen  temperature (             (L/L)         oxide______________________________________Example 30    250          >100          NDExample 31    250          >100          NDExample 32    200           404          NDExample 33    200          >100          NDExample 34    250          >100          NDExample 35    250          >100          NDExample 36    210           389          NDExample 37    220          >100          NDExample 38    280          >100          NDExample 39    250          >100          NDExample 40    230           381          NDExample 41    230          >100          NDExample 42    280          >100          NDExample 43    250          >100          NDExample 44    230           495          NDExample 45    230          >100          NDExample 46    260          >100          NDExample 47    260          >100          NDExample 48    250           406          NDExample 49    260          >100          NDExample 50    250          >100          NDExample 51    250          >100          NDExample 52    230          >100          NDExample 53    230          >100          NDExample 54    210          >100          NDExample 55    210          >100          NDComparative    300            16          NDExample 7Comparative    300            11          NDExample 8Comparative    400            25          NDExample 9Comparative    500            9           NDExample 10Comparative    350            12          NDExample 11Comparative    350            19          NDExample 12______________________________________

EXAMPLES 56 TO 88

Sponge zirconium and vanadium, molybdenum, titanium, chromium, tungsten, tantalum, niobium or tin each procured from the market in a total amount of about 500 g were blended in different compositions (40%, 50%, 70% or 90% by weight of Zr and the balance V, Mo, Ti, Cr, W, Ta or Nb or 50%, 70% or 90% by weight of Zr and the balance Sn)O and then melted by electron beam twice repeatedly to afford Zr-V, Zr-Mo, Zr-Ti, Zr-Cr, Zr-W, Zr-Ta, Zr-Nb or Zr-Sn alloys, which were crushed in a ball mill and screened to 14 to 20 mesh to prepare various alloys as cleaning agent each having a different composition from one another. Then 28.3 ml of each of the alloys was packed in a quartz-made cleaning column having 19 mm inside diameter and 400 mm length. Each of the Zr-alloys as the cleaning agent was tested in the same manner as in Example 7 to determine the temperature attaining 90% decomposition efficiency, cleaning capability and nitrogen oxide concentration. The results are given in Table 10.

COMPARATIVE EXAMPLES 13 TO 20

The procedure in Example 56 was repeated to carry out cleaning experiment except that there was used as the cleaning agent, V, Mo, Ti, Cr, W, Ta, Nb or Sn (1 to 2 mm in granule size) procured from the market. The results are given in Table 9 and Table 10, but the cleaning capability of Sn granule was not measured because of its failure to achieve 90% decomposition efficiency.

              TABLE 9-1______________________________________Alloy compo-     Lower limit of temperature (sition (wt %)    attaining ≧90% decompositionZr         other metal                efficiency______________________________________Example 56   40     V       60  235Example 57   50     V       50  205Example 58   70     V       30  170Example 59   90     V       10  175Example 60   40     Mo      60  250Example 61   50     Mo      50  190Example 62   70     Mo      30  180Example 63   90     Mo      10  180Example 64   40     Ti      60  205Example 65   50     Ti      50  195Example 66   70     Ti      30  165Example 67   90     Ti      10  170Example 68   40     Cr      60  230Example 69   50     Cr      50  235Example 70   70     Cr      30  200Example 71   90     Cr      10  205Example 72   40     W       60  195Example 73   50     W       50  185Example 74   70     W       30  180Example 75   90     W       10  185Example 76   40     Ta      60  185Example 77   50     Ta      50  180Example 78   70     Ta      30  175Example 79   90     Ta      10  190Example 80   40     Nb      60  225______________________________________

              TABLE 9-2______________________________________  Alloy compo-            Lower limit of temperature (  sition (wt %)            attaining ≧90% decomposition  Zr  other metal                efficiency______________________________________Example 81    50    Nb      50  195Example 82    70    Nb      30  175Example 83    90    Nb      10  185Example 84    50    Sn      50  200Example 85    70    Sn      30  195Example 86    90    Sn      10  200Example 87    70    V 15   Ti 15                      160Example 88    90    V 5    Mo 5 175Comparative     0    V      100  415Example 13Comparative     0    Mo     100  395Example 14Comparative     0    Ti     100  285Example 15Comparative     0    Cr     100  435Example 16Comparative     0    W      100  450Example 17Comparative     0    Ta     100  425Example 18Comparative     0    Nb     100  430Example 19Comparative     0    Sn     100  --Example 20______________________________________

              TABLE 10-1______________________________________  Cleaning   Cleaning capability                           Nitrogen  temperature (             (L/L)         oxide______________________________________Example 56    250          >100          NDExample 57    220          >100          NDExample 58    180          >100          NDExample 59    180          >100          NDExample 60    280          >100          NDExample 61    200          >100          NDExample 62    190          >100          NDExample 63    190          >100          NDExample 64    220          >100          NDExample 65    200          >100          NDExample 66    180          >100          NDExample 67    180          >100          NDExample 68    250          >100          NDExample 69    250          >100          NDExample 70    230          >100          NDExample 71    230          >100          NDExample 72    220          >100          NDExample 73    200          >100          NDExample 74    190          >100          NDExample 75    190          >100          NDExample 76    210          >100          NDExample 77    200          >100          NDExample 78    200          >100          NDExample 79    200          >100          NDExample 80    250          >100          ND______________________________________

              TABLE 10-2______________________________________  Cleaning   Cleaning capability                           Nitrogen  temperature (             (L/L)         oxide______________________________________Example 81    210          >100          NDExample 82    190          >100          NDExample 83    190          >100          NDExample 84    220          >100          NDExample 85    220          >100          NDExample 86    220          >100          NDExample 87    180          >100          NDExample 88    190          >100          NDComparative    450            23          NDExample 13Comparative    450            28          NDExample 14Comparative    300            41          NDExample 15Comparative    450            10          NDExample 16Comparative    470            21          NDExample 17Comparative    450            22          NDExample 18Comparative    450            34          NDExample 19Comparative    --           --            NDExample 20______________________________________

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for cleaning a harmful gas. More particularly, it pertains to a process for cleaning a nitrogen fluoride such as nitrogen trifluoride which is used or generated in the semiconductor manufacturing industry.

With the continuous development of the semiconductor industry, there has been a steady rise in recent years in the amount of nitrogen trifluoride which is used in the dry etching of silicon and silicon oxide, as a gas for cleaning the chamber of a CVD apparatus and the like. Nitrogen trifluoride gas is sparingly soluble in water and rather stable at room temperature with little reactivity with an acid or an alkali, but it is highly toxic and exerts an adverse influence on human bodies and the environment since the maximum permissible concentration thereof in the atmosphere is reported as being 10 ppm. It is therefore, necessary to clean a nitrogen trifluoride-containing gas after being used in a semiconductor production process prior to the discharge in the atmosphere.

In spite of its being stable at ordinary temperature, nitrogen trifluoride generates dinitrogen tetrafluoride, dinitrogen difluoride, dinitrogen hexafluoride, fluorine and the like due to heat, discharge, etc. in etching and cleaning processes, and each of them must be removed as well because of its toxicity is stronger than that of nitrogen trifluoride.

2. Description of the Related Art

As the method of removing a nitrogen fluoride contained in a mixed gas, there have heretofore been proposed (1) a process wherein the gas is brought into contact with metallic silicon at 100 (Japanese Patent Application Laid-Open No. 12322/1988), (2) a process wherein the gas is brought into contact with metallic titanium at 200 a process wherein the gas is brought into contact with Si, B, W, Mo, V, Se, Te, Ge or a non-oxide based compound of any of them at 200 800 wherein the gas is brought into contact with a metal halogenide capable of transhalogenation with nitrogen fluoride (Japanese Patent Publication No. 48569/1988), (5) a process wherein the gas is brought into contact with the oxide of a transition metal such as Fe, Mn or Cu at 250 higher (Japanese Patent Application Laid-Open No. 181316/1991), (6 ) a process wherein the gas is brought into contact with activated carbon at 300 237929/1987), (7) a process wherein the gas is brought into contact with a composition comprising as principal components Ni, Fe, Co or a noble metal such as Pt, Rh and Pd and at least one from alumina and silica at 200 27303/1987).

Nevertheless, any of the above-proposed processes is insufficient in gas cleaning capability and besides suffers the disadvantage as described hereunder. A volatile fluoride is produced in the processes (1), (2) & (3), a halogen such as chlorine is formed in, the process (4) and a nitrogen oxide is generated in the process (5), each incurring an expense in the treatment of itself. It is necessary in the processes (2) & (3) to heat the reaction system to 300 produced fluoride may not cover the surface of the reaction agent and thus hinder the cleaning reaction. The process (6) involves the danger of explosion due to a violent reaction at an elevated temperature and the problem of byproducing carbon tetrafluoride that is relatively stable and difficult to remove. In the process (7), a harmful gas is not byproduced but a high temperature is required for attaining sufficient cleaning capability and in the case of nickel (Ni), for example, for the purpose of achieving practical performance, the reaction system needs to be heated to 400 which the decomposition activity is insufficient and besides, the surface of the reaction agent is covered with the fluoride with the progress of the reaction, thereby failing to assure sufficient cleaning capability.

SUMMARY OF THE INVENTION

Under such circumstances, intensive research and investigation were continued by the present inventors on the development of a process for cleaning nitrogen fluoride with high treatment capacity at a low temperature without producing a harmful gas or a gas having a possibility of causing environmental pollution while solving the disadvantages of the conventional techniques. As a result, it has been found by the present inventors that the nitrogen fluorides can be removed in extremely high efficiency at a relatively low temperature without generating a substance adversely affecting the environment in the exhaust gas after a cleaning treatment by using zirconium or a zirconium-based alloy as the cleaning agent. The present invention has been accomplished on the basis of the above-mentioned finding.

Specifically the present invention provides a process for cleaning a gas containing a nitrogen fluoride as the harmful component which comprises bringing said gas under heating into contact with a cleaning agent comprising zirconium to remove said harmful component and a process for cleaning a gas containing a nitrogen fluoride as the harmful component which comprises bringing said gas under heating into contact with a cleaning agent comprising a zirconium-based alloy to remove said harmful component.

According to the present invention, the nitrogen fluorides including nitrogen trifluoride which is contained in the air, nitrogen, argon and hydrogen and other nitrogen fluoride such as dinitrogen tetrafluoride, dinitrogen difloride, dinitrogen hexafloride can efficiently be removed.

DESCRIPTION OF PREFERRED EMBODIMENTS

As the cleaning agent according to the present invention, there is employed zirconium or a zirconium-based alloy. As the zirconium source applicable to the preparation of the cleaning agent according to the present invention, mention may be made of metallic zirconium and sponge zirconium each available on the market, which can be used as such or by crushing them into an appropriate size. The zirconium available on the market sometimes contains hafnium in an amount of about 1 to 5% by weight, but such extent of hafnium content does not exert evil influence on the cleaning capability and is not liable to produce a harmful substance such as a volatile fluoride or nitrogen oxide in the course of cleaning reaction.

The zirconium-based alloy as mentioned above is usually the alloy of zirconium and at least one metallic element selected from the group consisting of iron, copper, nickel, aluminum, magnesium, calcium, zinc, lanthanum, cerium, vanadium, molybdenum, titanium, chromium, tungsten, tantalum, cadmium, yttrium, niobium and tin. The preferable alloy among them includes that of zirconium and at least one element selected from iron, copper, nickel, aluminum, magnesium, calcium, zinc, lanthanum and cerium, since it does not generate a volatile fluoride during,the course of reaction with a nitrogen fluoride and is easily available at a relatively low cost.

The nitrogen-fluoride removing function in the present invention is attributable principally to the zirconium component, which function is corroborated by the fact that the powdery substance formed during the cleaning operation as the reaction product of the cleaning agent and a nitrogen fluoride is proved to be mainly zirconium fluoride by the analysis of the substance. Accordingly, the cleaning agent consisting of zirconium as simple substance is characterized by its high cleaning capacity.

In addition, the cleaning capacity per unit weight of the cleaning agent consisting of a zirconium-containing alloy increases with an increase in the content of zirconium in the alloy. However, the alloying metallic component other than zirconium exhibits the effect on lowering the temperature at which a nitrogen fluoride is removed, and such effect is remarkably enhanced by an appropriate content of said component in the alloy. In more detail, the zirconium as simple substance necessitates a temperature of about 300 cleaning capability, whereas the zirconium-containing alloy enables a gas containing a nitrogen fluoride in the same concentration to be treated at a temperature lower than 300 example, practical cleaning capability is achieved at a lower temperature of 170 the alloying metal other than zirconium at 40% or less by weight.

The content of zirconium in the cleaning agent according to the present invention is not specifically limited, but in the case of zirconium-containing alloy, it is usually 20% or more, desirably 50% or more, more desirably 60% or more by weight with the above-mentioned metallic element as the balance. A content thereof less than 20% by weight results in insufficient capacity of removing nitrogen fluoride and decrease in the effect on lowering the cleaning temperature by alloying and besides causes a fear of byproducing a volatile fluoride depending upon the cleaning conditions.

The zirconium-based alloy can be produced by blending zirconium and the above-mentioned at least one metallic element at a prescribed blending ratio and subsequently alloying the resultant blend through electron beam melting, argon arc melting, high frequency heating melting or resistance heating melting each under vacuum or in an atmosphere of an inert gas or the like method of melting. The alloy thus produced is crushed to 6 to 20 mesh by mechanical crushing by means of a ball mill, jaw crusher, roll mill or the like to be employed as the cleaning agent. Alternatively, it is pulverized to about 100 mesh fine powder, which is made into granules or granulated powder having a size about 1 to 5 mm or molded into pellets to be employed as the cleaning agent. Various zirconium-based alloys available on the market may be employed as it is or after crushing to a suitable size.

The cleaning agent comprising zirconium or the zirconium-based alloy according to the present invention can be used as any of fixed bed, moving bed and fluidized bed. Under ordinary circumstances, the cleaning agent is packed in a cleaning column, and the gas containing a nitrogen fluoride is passed therethrough while being brought into contact with the cleaning agent so that the nitrogen fluoride as the harmful component is removed so as to clean the gas.

The temperature at which the gas to be treated is brought into contact with the cleaning agent (cleaning temperature) is usually 100 800 200 300 than 100 nitrogen fluoride, whereas that high than 800 disadvantages that stainless steel can not be used for the cleaning column, thereby lowering safety and besides increasing heating energy loss.

In the case where an atmospheric component is mixed in the gas to be treated, a high cleaning temperature is liable to cause heat release due to the reaction of the cleaning agent with oxygen and therefore, the cleaning operation is put into practice preferably at 250 lower.

The pressure during the cleaning operation is usually atmospheric pressure, but can be reduced pressure or raised pressure such as 1 kg/cm.sup.2 G.

There is no limitation to the flow velocity of the gas to be treated to which the cleaning process according to the present invention is applied, but in general the flow velocity is desirably lowered with increase in the concentration of the nitrogen fluoride contained in the gas to be treated.

The cleaning column is designed in accordance with the concentration of the nitrogen fluoride as the harmful gas, the amount and flow rate of the gas to be treated, etc. The space linear velocity (LV) in the column is preferably designed at 20 cm/sec or lower for a relatively low concentration of nitrogen fluoride such as 1000 ppm or less and at 5 cm/sec or lower for the concentration higher than 1000 ppm.

The length of the packed cleaning agent in the cleaning column varies depending on the flow rate of the gas to be treated, the concentration of the harmful gas and the like and can not be unequivocally specified, but is usually 50 to 500 mm, approximately from the practical viewpoint. In general, the length thereof is determined in accordance with the pressure loss through the packed bed, contact efficiency of the gas with the cleaning agent:, the concentration of the harmful gas and the like.

As mentioned hereinbefore, thereaction during the cleaning operation according to the present invention causes zirconium fluoride to be formed in the form of powder, which is discharged outside the system along with the treated gas or dropped to the bottom of the cleaning column, or allowed to remain in part in the packed zone of the cleaning agent depending on the operational conditions, whereby the pressure loss through the zone is unfavorably increased. Such increase in the pressure loss can be prevented by imparting vibration to the column continuously or intermittently by means of a vibrator or the like fixed to the column to drop the powder down to the bottom of the column. When necessary, a filter for collecting the powder in the treated gas may be installed on the downstream side of the cleaning column.

In summary, according to the process for cleaning a harmful gas of the present invention, nitrogen fluoride such as nitrogen trifluoride contained in a gas can efficiently be removed at a relatively low temperature without producing a harmful byproduct such as a nitrogen oxide. The process, therefore, exhibits an excellent effect on the cleaning of exhaust gas from a semiconductor manufacturing process or the like.

In the following the present invention will be described in more detail with reference to the non-limitative examples and comparative examples.